Theoretical Insights into the <sup>27</sup>Al NMR Parameters of Binary Aluminosilicate Glass and Their Relationship to the Atomic and Electronic Structure
Takahiro Ohkubo, Atsunobu Masuno, Eiji Tsuchida, Shinobu Ohki
Abstract
Al-rich 60Al 2 O 3 –40SiO 2 glass is a candidate for technological applications in electronic and optical devices. Though the amorphous structure of the glass has been studied using solid-state NMR and simulation approaches, the atomic and electronic structure have not been fully revealed. Solid-state 27 Al NMR spectra reflect the 27 Al environment, though a comprehensive understanding of the spectra and local structure is challenging when interpreting the broadened peak shapes of the amorphous state. Here, an accurate atomic structure of 60Al 2 O 3 –40SiO 2 glass was modeled using ab initio molecular dynamics (AIMD) simulations containing 418 atoms and employing the melt-quenching route with 15 K/ps. This simulation approach reproduced X-ray diffraction data better than classical molecular dynamics (CMD) simulations. The structure of the polyhedra formed by O bonded to Al was quantitatively analyzed by evaluating bond-angle distributions and the degree of symmetry using spherical harmonic functions. The relationship between chemical shifts and charge-balancing mechanisms was explored through the analysis of electronic structures obtained from AIMD-derived structures. Interestingly, the Al partial charge and the spatial electron distribution of Al–O bonds were independent of the Al coordination number, implying that valence electrons are not localized to specific atoms but are rather distributed throughout the glass network. The theoretical distribution of 27 Al NMR parameters was obtained through statistical analysis of theoretically calculated NMR parameters for 100 AIMD-derived structures. By comparing the experimental 27 Al NMR data with the theoretical distribution, the previously unclear relationship between 27 Al NMR parameters and local structure was elucidated.